1. Field of the Invention
The present invention relates to an exposure apparatus for producing a mastering disc for use to manufacture a recording media such as an optical disc or the like.
2. Description of Related Art
An optical disc as a recording media to record signals at a high density is produced by stamping, using an injection molding, lands and grooves formed on a mastering disc and corresponding to a signal to be recorded.
FIGS. 1A to 1C show the construction of an optical disc 1 produced as mentioned above. FIG. 1A shows the optical disc 1 having a signal recording area 2 defined on a disc substrate 3 made of an optically transparent plastic. Within the signal recording area 2, the disc substrate 3 has grooves 5 or a succession of pits 8 formed spirally on one side (signal side) 4 thereof at predetermined pitches P of 1 to 2 .mu.m. FIG. 1B shows the grooves 5, and FIG. 1C shows the pits 8. The grooves 5 and pits 8 are formed as periodically wobbled radially of the optical disc 1 as the case may be.
Normally, in an phase-change based optical disc or a magneto-optic disc, being a recordable optical disc, either the lands or grooves (lands 6, for example) formed on the signal side 5 by stamping them from a mastering disc are defined as a recording area, while the other (grooves 5, for example) are defined as a light reflecting area for tracking use. In case the grooves 5 or a succession of pits 8 are formed as wobbled, a periodic change of them is used as an address information over the entire optical disc 1. In a read-only optical disc, a succession of pits 8 on the signal side 4 are normally used as a recording area and tracking diffraction grating.
It should be noted that a recording layer made of a phase-change or magnetic film, a light reflecting layer, a protective layer, etc. are also formed on the signal side 4 on which the grooves 5 have also formed.
For recording or reproducing a signal into or from the optical disc 1, a laser light is irradiated from an optical pick-up to a reading side 7 of the optical disc 1 opposite to the signal side 4 while the optical disc 1 is being spun. If the optical disc 1 is of a recordable type, a signal is optically written into the recording layer on the land 6, for example, with the laser light irradiated onto the recording layer. Also, a signal optically written on the optical disc 1 is read with a laser light reflected from the optical disc 1.
Further, a tracking is controlled by detecting a reflected light from the groove 5, for example, to assure that a laser light for signal write and/or read can always be irradiated onto a predetermined track. In the case of a read-only optical disc, the laser light irradiated to the reading side 7 is detected as a reflected light and diffracted light from the signal side 4 on which a succession of pits 8 are formed to read a signal and control the tracking.
Since the performance of an optical disc as a recording media depends upon lands and grooves formed on the signal side of the optical disc, it has been required that the lands and grooves should be stamped with a high accuracy onto an optical disc from a mastering disc.
FIGS. 2A to 2C generally show together the process of forming a mastering disc for an optical disc.
First, a circular glass substrate 23 of which the surfaces have been sufficiently polished flat and washed is prepared as shown in FIG. 2A. Then, a photoresist made alkali-soluble when exposed to a light is applied to the glass substrate 23 as shown in FIG. 2B. Generally, this photoresist coating is done to a layer 20 of 0.1 .mu.m or so in thickness while the glass substrate 23 is being spun by a spinner.
Next, a signal recording laser light 31 is focused by a lens 32 onto the photoresist layer 20 as shown in FIG. 2C. At this time, while the glass substrate 23 is being spun, the laser light 31 can be shifted a predetermined distance per spin radially of the glass substrate 23 to form a latent groove image 33 spirally at predetermined intervals (track pitch P) on the photoresist layer 20. By irradiating the laser light 31 intermittently, a latent pit-train image 33 can be similarly formed on the photoresist layer 20. Further, by periodically deflecting the laser light 31 radially of the glass substrate 2, the latent image of grooves or succession of pits can be wobbled.
The exposure apparatus used here is called a laser cutting apparatus. It comprises a laser source to generate a laser light to which the photoresist layer 20 on the glass substrate 23 is exposed to record a signal, an optic modulator to modulate the laser light correspondingly to a signal to be recorded, an optical system to collimate the laser light onto the photoresist layer 20, an optical surface plate to turn the glass substrate and move the exposure position, etc.
FIGS. 3A to 3D show the process of developing in an alkaline developing solution an exposed glass substrate (subjected to a laser cutting process) as shown in FIG. 2C to remove, by melting with the laser light heat, a portion of the photoresist layer exposed to the laser light and thus made alkali-soluble.
FIGS. 3A and 3B show the developed glass substrates 23, respectively. FIG. 3A shows a mastering disc for a recordable optical disc having formed thereon grooves 25 resulted from the exposure to the laser light and melt-removal of the photoresist layer 20 and a land 26 remaining between the grooves 25. The grooves 25 and lands 26 are formed alternately radially of the mastering disc. FIG. 3B shows a mastering disc for a read-only optical disc having formed thereon a succession of pits 28 resulted from the exposure to the laser light and melt-removal of the photoresist 20. The pits 28 are formed repeatedly radially of the mastering disc.
As shown in FIG. 3C, the glass substrate 23 having grooves, lands or pits formed in the photoresist layer 20 through the development is plated with nickel to form a stamper 34 having grooves 25 or succession of pits 28 stamped thereon from the photoresist layer 20.
As shown in FIG. 3D, the grooves and lands or succession of pits of the stamper 34 are stamped to a plastic material for an optical disc substrate by an injection molding or the like method to form a disc substrate (replica substrate) 3 having the grooves 5 and lands 6 or a succession of pits 8.
It should be noted that the replica substrate 3 for a recordable optical disc has further a recording layer, a reflective layer, and a protective layer formed on the signal side 4 thereof where the grooves 5 have already been formed. Also, the replica substrate 3 for a read-only optical disc has further a reflective layer and a protective layer formed on the signal side 4 thereof where the pits 8 of the disc substrate 3 have already been formed.
In forming the above-mentioned mastering disc for an optical disc, an AOM (acousto-optic modulator) is used to select whether or not the photoresist 20 on the glass substrate 23 is exposed to a laser light. Also, an AOD (acousto-optic deflector) is used to deflect the exposure laser light for wobbling an exposure position of a photoresist layer radially of the optical disc. The construction and principle of operation of the AOM and AOD will be further discussed later.
FIG. 4 shows an example of optical system used in a conventional exposure apparatus incorporating an AOM and AOD.
The exposure apparatus has an optical path in which an AOM 62 is provided to modulate a laser light from an exposure laser source 52, and an optical path in which an AOD 58 is provided to deflect the laser light modulated by the AOM 55.
In this exposure apparatus, if no deflection of the laser light is intended, the laser light emitted from the laser source 52 is reflected by a reflecting mirror 60 provided on a stationary optical surface plate 50, focused by a lens 61, and modulated by the AOM 62. A divergent light after modulated by the AOM 62 is collimated by a lens 63, reflected by a beam splitter 64, passed through a half wavelength plate 65, passed through a polarizing beam splitter 66 on a moving optical surface plate 51 and a quarter wavelength plate 67, focused by a lens 68, and then focused by an objective lens 70 onto a glass substrate 71. The half and quarter wavelength plates 65 and 67 are provided to prevent the laser light from returning to the laser source.
If it is not intended to deflect the laser light in this exposure apparatus, the laser light emitted from the exposure laser source 52 is reflected by a beam splitter 53 provided on the stationary optical surface plate 50, focused by a lens 54, and modulated by the AOM 55. A divergent light after modulated is collimated by a lens 56, reflected by a reflecting mirror 57, deflected by the AOD 58 provided on the moving optical surface plate 51, reflected by a reflecting mirror 59 and polarizing beam splitter 66, focused by the lens 68 and them focused by the objective lens 70 onto the glass substrate 71.
A part of the laser light modulated by the AOM 62 is guided by the beam splitter 64 to a CCD 72 for monitoring.
FIG. 5 is a view, from the lateral side, of the optical system on the moving optical surface plate 51 of this exposure apparatus. As shown, the laser light deflected (wobbled) by the AOD 58 is focused by the lens 68, then reflected downward by a reflecting mirror 69, and focused by the objective lens 70 onto a spot 72 on the glass substrate 71.
It should be noted that since the glass substrate 71 is turned on a turn table at a predetermined speed, it is spirally exposed to the laser light by parallel moving the moving optical surface plate 51 radially of the glass substrate 71.
FIGS. 6A and 6B show in detail the shape of grooves in an optical disc produced by a conventional exposure apparatus provided with an optical system incorporating an AOD to deflect an exposure laser light.
FIG. 6A shows the shape of grooves 75 formed in a recordable magneto-optic disc of 64 mm in diameter, called "mini-disc". The grooves in this magneto-optic disc are resulted from wobbling of grooves (wide grooves) wider than those in normal optical discs. More particularly, the land is 0.5 .mu.m wide, groove is 1.1 .mu.m wide, and the track pitch P is 1.6 .mu.m. That is, the width of the grooves 75 is larger than the diameter of a focused laser spot 72 on a mastering disc. A mastering disc for forming the grooves 75 is turned more slowly than a mastering disc for a normal optical disc, and the focused laser spot 72 is deflected by the AOD over the width of the groove 75 to cover the entire width of the groove with the laser light.
The groove 75 has a shape periodically wobbled by deflecting the focused laser spot 72 radially of the optical disc by the AOD. The amplitude of the wobbling is as small as .+-.0.03 .mu.m. Also, the frequency of this wobbling is lower than the frequency of deflecting the laser light to cover the entire width of the groove, and the central value of the frequency of a signal (carrier frequency) to drive the AOD is changed periodically with this wobbling frequency.
FIG. 6B shows the shape of a groove formed in a magneto-optic disc of 88 mm in diameter called "HS disc" conforming to the ECMA (European Computer Machinery Association) Standard 239. In this magneto-optic disc, the wobbled succession of pits 78 is inserted in a space appearing periodically in the middle of a groove. The track pitch P is 1.2 .mu.m.
A mastering disc used to form the wobbled succession of pits has pits formed thereon by modulating an exposure laser light by the AOM (acousto-optic modulator) and deflecting the laser light by the AOD. The succession of pits 78 is thus wobbled correspondingly in the magneto-optic disc.
The AOM and AOD, acousto-optic effect elements, used in the exposure apparatus for preparing an mastering disc for an optical disc are disadvantageous in that if a period of refractive index compressional wave varies in a passage of an incoming laser light, an outgoing direction of first-order diffracted light will not be univocally be determinable so that the laser light will be diffracted in many directions at a same time. This phenomenon is called "cylindrical effect". The higher the frequency of laser beam deflection, or the larger the diameter of a laser beam passing through a crystal, the cylindrical effect will be correspondingly higher. The cylindrical effect will be further discussed later.
Furthermore, the AOM is disadvantageous in that if the beam diameter of the incoming laser light is larger, the frequency of modulation cannot be higher.
FIG. 7 shows a typical optical system designed to reduce the diameter of a laser beam passing through the AOM in order to avoid the above problem. That is, a laser light 90 is focused by a collimating lens 91. An AOM 92 is provided at the focal plane to reduce the diameter of a laser light through the AOM 92.
This optical system enables the AOM 92 to modulate a laser light at a high speed. However, the AOM 92 cannot change the traveling direction of the laser beam. Therefore, when this optical system is employed in an exposure apparatus, the exposure position of a laser light on a glass substrate cannot be changed, so that grooves and succession of pits cannot be wobbled.
FIG. 8 shows an example of optical system adapted to make a laser light incident upon the AOD, not focused, to thereby change the exposure position on a mastering disc, thus avoiding the above problem. When adopted in an exposure apparatus, however, this optical system will permit to change the exposure position of a laser light on a mastering disc, but the speed (frequency) of deflection at which no cylindrical effect takes place is low because the diameter of a laser beam incident upon the AOD is large. That is, the frequency of wobbling grooves or succession of pits on a mastering disc cannot be high, so that exposure of the entire mastering disc will take a longer time.
FIG. 9 shows an example of optical system used in a conventional exposure apparatus and adapted to modulate and also deflect an exposure laser light at a high speed to avoid the above problem.
In this exposure apparatus, there are provided an optical path for a laser light emitted from an exposure laser source 112, separated by a beam splitter 113 and reflected by reflecting mirrors 117 and 119, and an optical path for a laser light passing through the beam splitter 113, reflected by a reflecting mirror 120 and then by a beam splitter 124 and passing through a half wavelength plate 125. The laser beams traveling along these two optical paths are passed through a polarizing beam splitter 126 and a quarter wavelength plate 127, focused by a lens 128 and modulated by an AOM 114. A divergent light after modulated by the AOM 114 is focused by an objective lens 130 onto a glass substrate 131. The half wavelength plate 125 and quarter wavelength plate 127 are provided to prevent the laser light from returning to the laser source.
Also, a part of the laser light is guided by the beam splitter 124 to a CCD 132 for monitoring.
FIG. 10 is a view, from the lateral side, of the optical system on a moving optical surface plate 111 of the exposure apparatus. As seen, the laser light modulated by the AOM 114 is reflected downward by the reflecting mirror 129 and focused by the objective lens 130 onto a spot 132 on the glass substrate 131. It should be noted that since in this exposure apparatus, the AOM 114 is somewhat off the rear focal plane of the objective lens 128 to simultaneously attain a high speed modulation and deflection of the laser light.
It should also be noted that since the glass substrate 131 is turned on a turn table at a predetermined speed, it is spirally exposed to the laser light by parallel moving the moving optical surface plate 111 radially of the glass substrate 131.
However, when two laser beams resulted from splitting of a laser light from the exposure laser source 112 by the beam splitter 113 are used simultaneously, they cannot be modulated and deflected independently of each other.
FIG. 11 shows another example of optical system used in a conventional exposure apparatus and adapted to modulate and deflect an exposure laser light at a high speed to avoid the above problem.
The exposure apparatus employs an optical system using an infinite objective lens 160. The optical system has two optical paths. An AOM 152 is provided in one of the optical paths to modulate one of the two beams resulted from splitting by a laser light by a beam splitter 143, and an AOM 145 is provided in the other optical path to modulate the other of the two laser beams.
When no deflection of the laser light is intended in this exposure apparatus, the laser light emitted from the exposure laser source 142 is reflected by a reflecting mirror 150 provided on a stationary optical surface plate 140, and focused onto the AOM 152 which will modulate the laser light. A divergent light after modulated by the AOM 152 is collimated by a lens 153, reflected by a beam splitter 154, passed through a half wavelength plate 155, also passed through a polarizing beam splitter 156 and quarter wavelength plate 157 on a moving optical surface plate 141, and focused by the objective lens 160 onto a glass substrate 161. Note that the half and quarter wavelength plates 155 and 157 are provided to prevent the laser light from returning to the laser source.
When it is intended to deflect the laser light in this exposure apparatus, the laser light emitted from the exposure laser source 142 is reflected by a beam splitter 143 provided on the stationary optical surface plate 140, focused by a lens 144 and modulated by the AOM 145. A divergent light after modulated is collimated by a lens 146, reflected by a reflecting mirror 147, and deflected (wobbled) by an AOD 148 provided on the moving optical surface plate 141. Then, the laser light is reflected by a reflecting mirror 149, passed through the quarter wavelength plate 148 and focused by the objective lens 160 onto the glass substrate 161.
Also, a part of the laser light modulated by the AOM 152 is guided by the beam splitter 154 to a CCD 162 for monitoring.
FIG. 12 is a view, from the lateral side, of the optical system on the moving optical surface plate 141 in the exposure apparatus. As seen, the laser light deflected by the AOD 148 is reflected downward by the reflecting mirror 159, and focused by the objective lens 160 onto spots 162a and 162b on the glass substrate 161.
It should be noted that since the glass substrate 161 is turned on a turn table at a predetermined speed, it is spirally exposed to the laser beams by parallel moving the moving optical surface plate 141 radially of the glass substrate 161.
However, the conventional exposure apparatus is disadvantageous in that since the focus plane on which the AOM is to be placed is infinitely far, the optical system as shown in FIG. 7 cannot be applied to an optical system intended to reduce the diameter of the laser beam passing through the AOM as shown in FIG. 7.